doc: replace Mac OS with macOS

[carlescufi]

Apple's desktop operating system is called "macOS" and not "Mac OS". Fix the references to it in the documentation.

[dbkinder]

For the record, it was called "Mac OS X" and then changed to "macOS" :)

[carlescufi]

indeed, but I think we can now avoid using "Mac OS X" at all since it's been a while since they transitioned to the new name